Except for ripping, compressing etc. I'm not sure you'd see a lot of difference using the iMac dual core 3.06 with sufficient RAM than an octo with lots of apps open. Just sayin'.I have the 8 core and am very glad I went that route. I normally have 7 or 8 programs running at the same time and it definitely helps things work smoothly.
I have also watched the CPU Usage in Activity Monitor while ripping DVDs with Handbrake and usage goes as high as 720% - basically using 7 cores fully and partially using the 8th.
Once Grand Central is out OS X will do a far better job of juggling multiple cores as well. Most CPU intensive applications will be updated to take advantage of this as well so you can expect even better results.